The invention is related to a parabolic trough collector. The collector has a receiver formed by single absorber tubes that are installed on absorber tube supports and a parabolic reflector that reflects the solar radiation onto the receiver. The single absorber tubes of the receiver are surrounded by glass tubes. Special flexible unions are foreseen to compensate the heat expansion between the steal of the single absorber tubes and the glass tubes.
Commercial parabolic trough collectors for high temperature show high optical losses in the flexible unions between glass and absorber tubes. In those areas, where the absorber tube supports are supporting the receiver, the glass tube is interrupted and the flexible unions are installed. Therefore, the absorption of the concentrated solar radiation by an active absorber tube surface is not possible in those areas.
The one axis tracking parabolic trough collectors concentrate the solar radiation on a focus line where the receiver is situated. The heat transfer fluid, usually thermal oil or water, flows through the receiver. A big part of the solar radiation is converted by the receiver into thermal heat and is transferred to the heat transfer fluid. In order to avoid high thermal losses the single absorber tubes are surrounded by glass tubes. The space between absorber tube and glass tube is evacuated. Because of static reasons the absorber receiver consists on several single absorber tubes that are welded together in one line and in the welding area metal supports are installed that fix the single absorber tubes in the focus line of the parabolic trough collector. In operation conditions, the irradiated single absorber tubes become hot and expand more than the colder glass tubes. Therefore, in commercial parabolic trough collectors flexible unions are foreseen between absorber tubes and glass tubes. Normally, metal bellows are used to compensate the different expansions of the single absorber tubes and the glass tubes. The flexible unions are protected by aluminum sheets against the concentrated solar radiation that would produce dangerous high temperature gradients in the glass—metal welding area.
Commercial solar parabolic trough collectors for high temperature have high optical losses in the flexible union area. Because of the necessity of flexible bellows and the space for the absorber tube supports those areas cannot be used as active absorber area. About 4-6% of the reflected radiation is concentrated onto those non-active areas and lost for the system. The use of the concentrated radiation onto those non-active areas seems very complicated because of the continuously changing sun incident angle.